Conventionally, as a system for desalinating seawater, there has been known a seawater desalination system in which seawater passes through a reverse-osmosis membrane-separation apparatus to remove salinity from the seawater. In the seawater desalination system, the intake seawater is processed to have certain water qualities by a pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane-separation apparatus under pressure by a high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane-separation apparatus passes through a reverse-osmosis membrane against the reverse-osmosis pressure and is desalinated, and fresh water (permeate or desalted water) is taken out from the reverse-osmosis membrane-separation apparatus. The remaining seawater is discharged in a concentrated state of a high salt content as a reject (concentrated seawater) from the reverse-osmosis membrane-separation apparatus. The largest operational cost (electric expenses) in the seawater desalination system is energy cost for pressurizing the pretreated seawater up to such a pressure as to overcome the osmotic pressure, i.e. up to the reverse-osmosis pressure. That is, the operational cost of the seawater desalination system depends heavily on pressurizing energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric expenses as the highest cost in the seawater desalination plant are consumed to operate the high-pressure pump for pressurizing the seawater. Therefore, pressure energy possessed by the high-pressure reject (concentrated seawater) with the high salt content which has been discharged from the reverse-osmosis membrane-separation apparatus is utilized for pressurizing part of the seawater. Therefore, as a means for utilizing the pressure energy of the concentrated seawater (brine) discharged from the reverse-osmosis membrane-separation apparatus to pressurize part of the seawater, there has been utilized an energy exchange chamber in which an interior of a cylinder is separated into two volume chambers by a piston arranged to be movable in the cylinder, a concentrated seawater port is provided in one of the two separated spaces to introduce and discharge the concentrated seawater, and a seawater port is provided in the other of the two separated spaces to introduce and discharge the seawater.
FIG. 25 is a schematic view showing a configuration example of a conventional seawater desalination system. As shown in FIG. 25, seawater pumped into the seawater desalination system by an intake pump (not shown) is processed to have certain water qualities by a pretreatment system, and then the pretreated seawater is delivered via a seawater supply line 1 into a high-pressure pump 2 that is directly coupled to a motor M. The seawater which has been pressurized by the high-pressure pump 2 is supplied via a discharge line 3 to a reverse-osmosis membrane-separation apparatus 4. The reverse-osmosis membrane-separation apparatus 4 separates the seawater into concentrated seawater with a high salt content and fresh water with a low salt content and obtains the fresh water from the seawater. At this time, the concentrated seawater with a high salt content is discharged from the reverse-osmosis membrane-separation apparatus 4, and the discharged concentrated seawater still has a high-pressure. A concentrated seawater line 5 for discharging the concentrated seawater from the reverse-osmosis membrane-separation apparatus 4 is connected via a directional control valve 6 to a concentrated seawater port P1 of an energy exchange chamber 10. A seawater supply line 1 for supplying the pretreated seawater having a low pressure is branched at an upstream side of the high-pressure pump 2 and is connected via a valve 7 to a seawater port P2 of the energy exchange chamber 10. The energy exchange chamber 10 has a piston 12 therein, and the piston 12 is arranged to be movable in the energy exchange chamber 10 while separating an interior of the energy exchange chamber 10 into two volume chambers.
The seawater pressurized by utilizing a pressure of the concentrated seawater in the energy exchange chamber 10 is supplied to a booster pump 8. Then, the seawater is further pressurized by the booster pump 8 so that the seawater has the same pressure level as the discharge line 3 of the high-pressure pump 2, and the pressurized seawater merges via a valve 9 into the discharge line 3 of the high-pressure pump 2 and is then supplied to the reverse-osmosis membrane-separation apparatus 4.
This kind of seawater desalination system and energy exchange chamber is described in, for example, U.S. Pat. No. 5,306,428, U.S. Patent Laid-Open Publication No. 2006-0151033, U.S. Pat. No. 7,168,927, and the like.
In the energy exchange chamber 10, the directional control valve 6 is switched to a discharge side for discharging the concentrated seawater to suck the seawater of the seawater port P2, and the seawater flows from the seawater port P2 into the energy exchange chamber to move the piston 12 to a side of the concentrated seawater port P1. In this state, the energy exchange chamber 10 is nearly filled with the seawater. Then, the directional control valve 6 is switched to a supply side for supplying the high-pressure concentrated seawater to the energy exchange chamber 10, and the piston 12 moves toward a side of the seawater port P2 so as to push the seawater which has flowed into the energy exchange chamber 10, thereby supplying the seawater to the booster pump 8 through the valve 7 at a side of the seawater port P2.
The valve 7 at the side of the seawater port P2 is composed of a well-known fluid machine such as a check valve or a directional control valve so that the high-pressure fluid flows to the booster pump 8 and the low-pressure fluid flows to the energy exchange chamber 10.
Since the booster pump 8 pressurizes the seawater which has been already pressurized by the energy exchange chamber 10, to the same pressure level as the high-pressure pump 2, the booster pump 8 can be driven by a small amount of energy. Specifically, the flow rate of the seawater supplied to the reverse-osmosis membrane-separation apparatus 4 becomes the flow rate obtained by adding the flow rate of the seawater from the energy exchange chamber 10 to the flow rate of the seawater from the high-pressure pump 2, and thus the flow rate of the seawater to be treated in the entire system becomes large. Because the seawater supplied from the energy exchange chamber 10 has been pressurized by utilizing energy of the high-pressure concentrated seawater, input energy in the entire system can be reduced. In other words, the system which can reduce capacity and driving energy of the high-pressure pump to obtain the same flow rate of the seawater to be treated can be constructed.
The above-described conventional energy exchange chamber is suitably selected in its size and in number depending on the capacity (flow rate) to be treated in the seawater desalination system, and the conventional energy exchange chamber is generally in the form of an elongated cylinder having a large diameter and has a piston arranged to be movable in the cylinder so that an interior of the cylinder is separated into two volume chambers.
FIG. 26 is a cross-sectional view showing a configuration example of a conventional energy exchange chamber 10. As shown in FIG. 26, the energy exchange chamber 10 comprises a cylinder 11 having a cylindrical shape, a piston 12 which is reciprocated in the cylinder 11, and flanges 13 for closing both opening ends of the cylinder 11. The flanges 13 are fixed to flange portions 11f of the cylinder 11 by bolts 14 and nuts 15, and a concentrated seawater port P1 is formed in one of the flanges 13 and a seawater port P2 is formed in the other of the flanges 13.
In this case, for the purpose of improving sliding characteristics of the piston 12 against the inner wall of the cylinder, sliding rings 16 are mounted on a cylindrical surface of the cylindrical piston 12. The sliding ring 16 is composed of a material having a low friction and excellent wear resistance, and, for example, engineering plastics or the like is selected. The piston 12 is always reciprocated in the chamber because the seawater flows into the chamber and is then pushed out by the concentrated seawater. Therefore, even if the piston 12 is composed of a material having excellent wear resistance, the piston 12 is eventually worn and required to be replaced. Further, the piston 12 is reciprocated in the chamber, and hence it is difficult to grasp wear status. When the sliding seal 16 is worn, a metal part of the piston 12 is brought into direct contact with a metal part of the cylinder 11 to cause damage to each of the metal parts. According to circumstances, the chamber itself must be replaced.
Further, an inner diameter of the energy exchange chamber is required to be an uniform cylinder so as to fit with an outer diameter of the piston (outer diameter of the sliding seal). Therefore, when the chamber becomes long as much as several meters, it is difficult to process the inner diameter of the chamber, and eventually the chamber itself becomes a very expensive product.